1. Field of the Invention
The present invention relates to an electrochemical device and a process for producing an electrochemical device.
2. Related Background Art
With the development and prevalence of portable electronic instruments, research is increasingly focused on secondary (or rechargeable) batteries such as lithium ion batteries having a high electric capacity capable of continuous operation for a long time and electrochemical devices such as electric double layer capacitors. Such an electrochemical device has a structure composed of electrodes that are stacked one on another with an electrolyte intervening between them. The electrode is composed of a current collector on the surface of which an electrode layer containing an active material is formed. The electrolyte is composed of a separator impregnated with an electrolyte.
In the electric double layer capacitor, a metal foil of aluminum, nickel, copper or stainless steel etc. or an electrically conductive resin is used as a material for the current collector. As an active material, an activated carbon that has a large specific surface area is generally used. The electrode is made by forming the electrode layer directly on the current collector. For example, Japanese Patent Application Laid-Open No. 4-162510 and Japanese Patent Application Laid-Open No. 8-55761 discloses electric double layer capacitors in which an electrode layer is formed by directly applying electrode coating liquid, which is in the form of slurry that is formed by mixing an active material and an electrically conductive auxiliary material with a solvent in which a binder resin is dissolved, to the surface of a current collector and thereafter drying it.
In the current collector that uses a metal foil as described above, there is a problem in adhesivity of the metal foil with the electrode layer, which sometimes causes an increase in the internal resistance or detachment of the electrode layer from the current collector, so that cycle properties would be deteriorated when charging and discharging are performed repeatedly. Especially, current collectors made of aluminum foils noticeably raise the problem of insufficient adhesivity with the electrode layer.
In addition, when the electrode layer is formed by applying electrode coating liquid to the collecting layer directly, it is necessary to increase the amount of the binder resin in order to enhance the adhesivity of the electrode layer to the current collector. This inevitably leads to a decrease in energy density.
On the other hand, there is also known electric double layer capacitors in which the electrode layer and the current collector are bonded to each other by providing an adhesive resin layer between them, instead of directly forming the electrode layer on the current collector. By bonding the electrode layer and the current collector with the adhesive resin, adhesivity of the electrode layer and the current collector can be enhanced. For example, Japanese Patent Application Laid-Open No. 6-53079 discloses an electric double layer capacitor in which an electrode layer that has been formed in advance is bonded to a current collector that also serves as an outer packaging material with an electrically conductive adhesive resin. In addition, Japanese Patent Application Laid-Open No. 2000-252175 discloses an electric double layer capacitor that is formed by providing a material that shows a fluid nature under application of heat, between the electrode layer and the current collector, applying heat under pressurization, and thereafter cooling it to bond the electrode layer and the current collector. In this electric double layer capacitor, the electrode is made by applying an adhesive resin made of powdered, meshed or porous polyethylene or the like to the current collector, and thereafter laminating the electrode layer. Then, a plurality of electrodes thus made are stacked one on another with a separator intervening between each two adjacent electrodes, and heat is applied from both sides of the electrodes under pressurization by a pressuring jig. The electrodes are maintained under this state for a while and then cooled down to room temperature. Thus, the electrode layer and the current collector are bonded to each other.
However, in the case in which the electrode and the current collector are bonded by an adhesive resin, there is a problem that the energy density is decreased due to the presence of thickness of the adhesive resin layer, which is necessitated by use of the adhesive resin. In addition, if an insulating material is used as a material for the adhesive layer, it is necessary to form the adhesive resin layer only in a limited area on the surface of the current collector, in view of an increase in the internal resistance. Therefore, it is difficult to provide sufficient adhesivity.
On the other hand, in the case in which an electric double layer capacitor is produced by providing an adhesive resin that shows a fluid nature under application of heat, heating it under pressurization and then cooling it, a complex manufacturing apparatus and a complex manufacturing process that includes heating under pressurization and cooling are required.
The above-described problem of insufficient adhesivity of an electrode layer and a current collector is shared not only by electric double layer capacitors but also by electrochemical devices such as secondary batteries that have structures including electrodes stacked with separators between. Therefore, in electrochemical devices having such structures, problems such as an increase in the internal resistance and deterioration in the energy density would arise, when an adhesive resin layer is provided in order to ensure adhesivity of an electrode layer and a current collector.